Add support for environment variables.

[guillermoandrae]

If I'm using this tool on the same machine for every build, I'd like to be able to set environment variables instead of having to explicitly pass the following options (or use a wrapper script):

--target-branch
--access-token
--cache-dir
--token-file
--github-api

[guillermoandrae]

And, to be clear, if an option is included, the value assigned to it should override the value from the appropriate environment variable.